A Trade-Off Between Wear Resistance and Corrosion Protection in Sol–Gel SiO2–Al2O3 Hybrid Coatings on AISI 316L

Abstract

In this study, single-phase (SiO2 and Al2O3) and hybrid (SiO2–Al2O3) ceramic coatings were synthesized on 316L stainless steel via a sol–gel dip-coating route followed by thermal densification at 800°C for 1 h. Cross-sectional FE-SEM observations confirmed continuous, well-adhered coatings with uniform thicknesses ranging from 1.26 µm to 3.92 µm. Microstructural characterization revealed that incorporating alumina into the amorphous siloxane matrix induced the crystallization of hard γ-Al2O3 phase domains. Oliver–Pharr nanoindentation and Hertzian elastic contact stress analysis (1.6–2.0 GPa under 1 N) verified substantial mechanical reinforcement without macro-plastic substrate deformation. Tribological evaluations demonstrated that the 50% Al2O3–50% SiO₂ hybrid coating achieved superior wear resistance. Conversely, electrochemical evaluations via potentiodynamic polarization (Tafel) and Electrochemical Impedance Spectroscopy (EIS) revealed an opposite trend for corrosion protection. Single-phase 100% Al2O3 and 100% SiO2 coatings provided exceptional dielectric barrier protection, with ultra-low normalized corrosion current densities and high pore and charge-transfer resistances. In contrast, thermal expansion mismatch in the 50/50 hybrid induced an interconnected microcrack network, increasing apparent porosity and degrading Rc and Rct. Overall, these findings establish a fundamental microstructural trade-off in sol–gel oxide coatings between mechanical wear resistance and long-term electrochemical barrier integrity.